You should be able to force subscriber (MSISDN) to specific technology. You can't see if subscriber has set 3G in his device as only technology. Usually there is 2G, 2G/3G, 2G/3G/4G. There could be a problem from the network side that doesn't allow subscriber to get on 4G. Also, subscribers, when they have poor 4G coverage and have some skills, set their phones to 2G/3G only. Only what you can do is track subscriber's mobility. If subscribers moves in larger areas and all cells are 3G than it might be locked to 3G but you wouldn't know the reason. You would need to check polices and see if there is a rule forcing device to 3G. If not, you can conclude that subscriber is forcing himself on 3G. If subscriber is not always on 2G/3G, rot doesn't move from home cell you can't assume that device is locked to 3G.